While Gaspar is completely correct, you can't always ask for that full amount. Usually when you are in a hotel, your breakfast is included. Many times the conference will also give the interpreters lunch on site, especially if the breaks are a bit tight to go out and find a place to eat. In this case, you can't really ask for more than half, or 50 euros. If somehow your evening meal is also covered, you could still ask for a minimum amount of money for incidentals that you wouldn't need if you were living at home - buying some aspirin, for example.

For anyone who needs US rates for US tax purposes, the IRS uses their own per diem rates for domestic per diems on this webpage: link text

and for per diems outside the US uses the State Department per diem rates on this webpage: link text. They are broken down into hotel costs, and meals and incidentals.

As a note of interest, when accompanying a summit delegation, in the past at least, we were allowed 150% of the per diem, as we had much less leeway as to where we stayed, ate, etc.

Just to give you an idea... the EU pays 94 euro as a per diem (on top of that it refunds a hotel on presentation of the bill). If the meeting organisers are providing meals around 30 euro is deducted from the 94 euro for each meal. If the hotel bill includes breakfast a further 10 euro are deducted from the 94.

And don't forget that your perdiem covers your hotel + 3 meals + extras AND airport transfers. If you leave very early in the morning or arrive late at night, you'll take a taxi or two (or even four in some cases, depending on your flight schedule) and, in Europe at least, this should also be covered by your perdiem.